Large aircraft often use an on-board auxiliary power unit (APU) to provide electrical power and compressed air for systems throughout the airplane. When the aircraft is grounded, the APU provides the main source of power for environmental control systems, hydraulic pumps, electrical systems and main engine starters. During flight, the APU can supply pneumatic and electric power.
Auxiliary power units are generally small gas turbine engines, often mounted in the aft tail section of the aircraft. They require a certain amount of cooling air, and are lubricated by oil that is generally cooled by an oil cooler which also requires cooling air. Active cooling systems are usually employed to provide this cooling air, and are typically comprised of an active fan used to push air through the oil cooler and across auxiliary power unit components. These fans are driven at high speeds by the APU through a complex shaft and gear assembly. The mechanical complexity and high operating speeds of these fans increases the possibility of failure. Active fan cooling systems therefore can significantly reduce the reliability of an auxiliary power unit.
While APU passive cooling systems which eliminate the need for active fan cooling systems are well known, they all generally draw cooling air into the APU compartment, before it is drawn through the air cooled oil cooler. This arrangement causes the cooling air to be heated up in the compartment before it reaches the oil cooler, and therefore, oil cooling is not optimized. U.S. Pat. No. 5,265,408, for example, discloses a method and apparatus for cooling a compartment mounted gas turbine engine comprising a first exhaust eductor within which is mounted an oil cooler, and which incorporates a mixer nozzle to entrain cooling air flow first through the APU compartment and then through the oil cooler. Surge bleed flow from the load compressor is discharged into the exhaust eductor. Ambient air is received into the compartment through a second exterior eductor inlet.
U.S. Pat. No. 5,655,359 similarly discloses an APU passive cooling system wherein cooling air for the oil cooler is drawn from the compartment. An inlet scoop in the engine air intake duct used to divert a portion of the air flow into the APU compartment. This air is used to cool the engine before being drawn through the oil cooler, mounted in a vacuum duct, by a lobed mixer which acts as an aspirator.
U.S. Pat. No. 6,092,360 discloses an APU passive cooling system in which cooling air is drawn into the engine compartment through an opening located in the rear of the aircraft. An eductor mounted before the exhaust duct of the engine, draws compartment air through the oil cooler, which in turn draws atmospheric air in through the aft opening.
Thus, while these patents provide for cooling of an auxiliary power unit without the use of a mechanically driven fan, they all teach systems which draw cooling air for the oil cooler from the APU compartment. A need exists for an auxiliary power unit passive cooling system that can provide enhanced oil cooling capabilities by directing exterior cooling air, through ducts, directly to the oil cooler, and which is nevertheless adaptable enough to be able to provide damage protection from foreign objects and be combined with the engine compressor surge bleed flow to provide improved airflow through the oil cooling heat exchanger.